The so-called hierarchy encoding signifies a technology of hierarchically encoding image data in an order of coarse information to fine information. Encoding an image hierarchically makes it possible to cope with terminals each having a different display resolution and a different a transfer environment on the condition alone that partial addition/deletion is performed for one piece of compressed data. It is thinkable that with a development in Internet, and diversity of an image reproduction environment, the technology of hierarchically encoding an image becomes of importance increasingly.
On the other hand, so as to realize a high-efficient image encoding, the prediction decoding process, which is performed by making a reference to surrounding pixels, becomes complicated increasingly. The movement compensation prediction in the latest international standard of the moving image coding scheme, namely, H.264/MPEG-4AVC differs in the following points as compared to that of MPEG-2.
(1) A block, which becomes a unit for a compensating process, can be sub-divided into 4 times 4 blocks.
(2) A precision of movement information can be fined at a level of being as fine as one-fourth of a pixel or so.
(3) A filter for interpolating a pixel value at a decimal point, which is a six-tap filter, is lengthened.
Further, with the process of, in intra-frame coding, predicting an intra-block pixel value from the neighboring pixels, H.264/MPEG-4AVC adopts the process of not only copying the pixel value in a longitudinal direction and a traverse direction, but also interpolating the pixel value in an oblique direction.
In FIG. 4, a configuration is shown of the hierarchical image decoding device in a pyramidal coding scheme, being a general technique of the hierarchy encoding, which is adopted in MPEG-2 as well.
At first, an entropy decoding unit 101, an inverse quantizing unit 102, and an inverse frequency converting unit 103 perform a process for lower hierarchical data 1001, respectively, and create a prediction error signal 1004. A prediction decoding unit 105 performs a prediction decoding process by making a reference to a lower hierarchical decoded image 1005 filed into a memory 104, and creates a prediction signal 1007. The prediction signal 1007 and the prediction error signal 1004 are added and a lower hierarchical decoded image 1009 is created. The lower hierarchical decoded image 1009, which is filed in the memory 104, is employed for later decoding.
Next, an entropy decoding unit 201, an inverse quantizing unit 202, and an inverse frequency converting unit 203 perform a process for upper hierarchical encoding data 2001, respectively, and create a prediction error signal 2004. A prediction decoding unit 205 performs a prediction decoding process by making a reference to a lower hierarchical decoded image 1010 filed into the memory 104, and an upper hierarchical decoded image 2005 filed into a memory 204, and creates a prediction signal 2007. The prediction signal 2007 and the prediction error signal 2004 are added, and an upper hierarchical decoded image 2008 is created. The upper hierarchical decoded image 2008, which is filed in the memory 204, is employed for later decoding.